Bipolar transistors are important components in, for example, logic circuits, communications systems, and microwave devices. A bipolar transistor is essentially a three terminal device having three regions, an emitter, base and collector region, wherein the emitter and collector regions are of one conductivity type and the base is of another.
Since the advent of bipolar transistors, many attempts have been made to improve the performance of the transistor. Some of these attempts have focused on increasing the current gain by improving the injection efficiency of the minority carriers from the emitter to the base. In order to accomplish this, wide bandgap transistors have been fabricated wherein the bandgap of the emitter is wider than the bandgap of the base. The wide bandgap transistor has been described in many publications and patents, some of which include Herbert Kromer's "Theory of Wide Gap Emitter for Transistors" reported in The Proceedings of IRE, 1957, p. 1535; U.S. Pat. Nos. 2,569,347; and 4,716,445.
In U.S. Pat. No. 4,716,445 entitled "Heterojunction Bipolar Transistor Having A Base Region Of Germanium", and assigned to NEC Corporation, a wide bandgap heterojunction bipolar transistor is described. The transistor includes a collector region of gallium arsenide, a base region of germanium and an emitter region having a semiconductor layer of mixed crystal of silicon and germanium. The mixed crystal semiconductor layer may have a uniform distribution of silicon in germanium or a graded distribution of silicon. If the graded distribution is provided, the content of silicon increases with distance from the interface between the base and emitter regions. The silicon is added to the emitter region in order to increase the bandgap of the emitter region so that it is larger than the base bandgap. With the above described structure of a wide bandgap transistor and the fact that germanium has a large electron mobility, performance of the transistor is enhanced.
Many attempts in improving transistor performance have focused on, for example, decreasing the width of the base region of a transistor and decreasing the base transit time for a constant base width. In one example, in order to decrease both the base -width and the base transit time, a bipolar transistor having a graded silicon/germanium base is fabricated. The silicon/germanium base is epitaxially grown on an N-type collector, which is formed on a silicon substrate. Subsequently, the emitter region is formed on the base. In order to form the emitter, a window is opened and polysilicon is deposited. The polysilicon is heavily doped with an N-type dopant, such as arsenic. For example, the arsenic is implanted in the polysilicon and then a high temperature anneal cycle (e.g. 850.degree. C. for 15 minutes) is used to diffuse the arsenic through the polysilicon, the polysilicon/silicon interface of the emitter and into the base. This process enables bases of approximately 600 Angstroms to be formed.
However, the above-described process places a limitation on how thin the bases can be fabricated. This limitation is due to the fact that the high temperature anneal cycle causes the P-type dopant, e.g. boron, in the base to diffuse thereby causing the base width to increase. Therefore, there is a limitation as to how thin the bases can be fabricated when the emitter is formed and contacted by polysilicon.
In order to fabricate transistors with thinner bases, it has been suggested to eliminate polysilicon emitter contacts and to use single-crystal emitters contacted by metal. A problem associated with single-crystal emitters, however, is that in order to obtain a reasonable current gain, the single-crystal emitter must have an emitter thickness greater than 300 Angstroms (for instance, 2000 Angstroms) and, therefore, the emitter charge storage is very large causing the device to be slow.
Accordingly, a need exists for a transistor having a single-crystal emitter with reduced charge storage. In addition, there still exists a need to increase device performance. Further, a need still exists to reduce the width of a base region in the device.